"Animal Liberation" by Peter Singer, which argues against speciesism and the ethical treatment of animals, as well as "Eating Animals" by Jonathan Foer, which delves into the moral complexities of eating animals and factory farming. Both these books have convinced me to go vegan. I've been vegan for a decade now and don't regret it one bit.
As a side effect, I've also become more health conscious, because a strict vegan diet doesn't provide everything, so I did a lot of research into what I'm eating, what my body needs (and doesn't need) etc. As a result I feel like my health has improved a lot - my hairloss has mostly stopped, my complexion has improved, also I used to have a skin condition which is now under control, no depression episodes, and I rarely fall sick.
It's been an ongoing process of learning though. Most recently I've found out about Choline, which has a critical role in neurotransmitter function and affects your mood, and thankfully I found that my diet already has enough Choline in it, so it wasn't a worry or anything. But it's always interesting knowing what's in what your eating, things your body needs etc.